1. Field of the Invention
The present invention relates generally to a glass antenna device for use on vehicles having a rear window in a hinged rear hatch or a hatchback which is widely used in recreational vehicles, station wagons and so on. More particularly, this invention relates to a vehicle window glass antenna device having a first antenna arranged in a rear window glass for the reception of FM and TV broadcasts, and a second antenna arranged in a window glass of a fixed side window, such as an opera window or a quarter window, for the reception of AM broadcasts.
2. Description of the Related Art
Glass antenna devices including antenna strips provided, together with a plurality of defogging heater elements (forming a defogger), on a rear window glass of a vehicle for the reception of radio waves are known. In the known glass antenna devices, a choke coil is inserted in a power supply line to the heater elements so that the defogging heater elements can be utilized as a receiving antenna. Additionally, the configuration, arrangement and position of the antenna strips are adjusted so that radio waves in an AM broadcast band and an FM broadcast band (as well as a TV broadcast band when the need arises) can be received at a high sensitivity.
Some known vehicle window glass antenna devices for the reception of AM-FM broadcasts include a preamplifier provided between a feed terminal of the antenna strips and an input terminal of the radio set or receiver so as to improve the reception performance.
However, use of the preamplifier poses a problem in that waveform distortion or cross modulation distortion is likely to occur during reception in a strong electric field, and noise mixed in the glass antenna is amplified. To cope with this problem, various improvements have been proposed for the glass antenna devices to obtain a sufficient degree of reception sensitivity without any preamplifier.
One such proposed improvement is disclosed in Japanese Patent Laid-open Publication No. (HEI) 6-268422, which provides a glass antenna device having a loop-shaped main antenna strip and a feeding point provided on a vehicle rear window glass. Plural return strips extending toward the center of the main antenna strip are provided in the neighborhood of the both sides of the main antenna strip, with the top ends of the return strips used as open ends, so as to obtain a sufficient reception sensitivity in the FM broadcast band without using a preamplifier.
Japanese Patent Laid-open Publication No. (HEI) 7-111412 discloses another improved known vehicle glass antenna device which comprises a defogger composed of a plurality of defogging heater elements formed of electric conductors provided on a vehicle rear window glass, an antenna conductor arranged in a predetermined pattern in the neighborhood of the defogger so as to form a capacitive coupling together with the defogger, a reactance circuit inserted between the defogger and a DC power source, and a matching circuit inserted between a feeding point of the antenna conductor and a receiver. To receive a broadcast at a high sensitivity without using a preamplifier, an anti-resonance point is set to frequencies at the outside of the broadcast band by the stray capacitance of the defogger and the reactance circuit, and a resonance point is set between a minimum frequency of the broadcast band and a frequency being a multiple of 1.5 of a highest frequency of the broadcast band by the impedance of the matching circuit, the impedance of the receiver, and the impedance when viewing the antenna conductor from the matching circuit.
Still another improved vehicle glass antenna device known from Japanese Patent Laid-open Publication No. (HEI) 8-162826 includes plural heating conductor strips provided on a vehicle rear window glass in the horizontal direction, and plural antenna conductor strips arranged in the horizontal direction. An auxiliary strip is provided close to a conductor strip at the lowest part of the antenna between a lowest antenna strip and an uppermost heating conductor strip arranged in the horizontal direction. A coil or a coil and a capacitor are inserted between the auxiliary strip and the uppermost heating conductor strip. This arrangement can suppress a leakage current to the vehicle body during the reception of an AM or an FM broadcast wave, and also realizes optimum tuning without any restriction onto an antenna pattern for the reception of FM broadcast waves.
In a vehicle glass antenna device disclosed in Japanese Patent Laid-open Publication No. (HEI) 9-107218, a defogger, an antenna conductor for the AM frequency band, and an antenna conductor for the FM frequency band are provided on a vehicle rear window glass. The AM antenna conductor is capacitively coupled to the defogger. The AM antenna conductor and the FM antenna conductor are connected by a circuit component including the inductance component, and a low pass filter is inserted between the defogger and a DC power source. This arrangement can eliminate the need for a choke coil and improves the S/N ratio.
A vehicle glass antenna device proposed by the present assignee by way of Japanese Patent Laid-open Publication No. (HEI) 9-18222 includes an exclusive antenna and a compatible antenna (defogging heater conductor) formed on a window glass. A transformer has a primary winding connected at its one end to the exclusive antenna and at its middle point to the compatible antenna, and a secondary winding connected to a feeder cable to perform the impedance conversion. With this arrangement, a choke coil is no longer needed, the capacity of the feeder cable is reduced when viewed from the antennas, and the transmission loss is also reduced to such an extent that a practically sufficient reception sensitivity can be obtained.
Somewhat different vehicle glass antenna devices disclosed in, for example, Japanese Patent Laid-open Publications Nos. (HEI) 2-39702, 6-224611 and 6-224612 have an antenna strip for the reception of FM broadcast waves and another antenna strip for the reception of AM broadcast waves, both antenna strips being provided on a vehicle side window glass.
It appears clear from the foregoing description that for the reception of an AM broadcast at high sensitivity without use of a preamplifier, an AM broadcast receiving antenna formed on a vehicle rear window glass requires a choke coil or a low pass filter inserted in a feed path or line to a defogging heater or defogger which is used in combination with a matching circuit or an impedance conversion transformer inserted between an antenna strip and a feeding point, or with a circuit including an inductance component or a capacitor between the antenna strip and the defogger or between the antenna strip and an auxiliary strip. Thus, a space must be provided in the vicinity of the rear window glass or on a surface of the rear window glass for the installation of the circuit including the choke coil, inductance component, capacitor and so on.
However, some types of vehicles, due to the structure peculiar thereto, are sometimes unable to provide a sufficient space for installation of the circuit components. Particularly, most vehicles having a hinged rear hatch (hereinafter referred to, for brevity, as "hatchback vehicles") have an insufficient circuit-components installation space.
The hatchback vehicles further require the circuit to have circuit components and a packing structure which are strong enough to withstand shock or impact force produced when the hatchback is opened and closed. This requirement renders the packaging process uneasy to achieve and induces additional cost.
Furthermore, in the antenna formed on the rear window glass for the reception of AM broadcasts, noise generated by various electric equipment such as a rear wiper, rear lamps and indicators are likely to be mixed in, and an appropriate countermeasure to the noises is in many cases difficult to achieve.
In the case where an antenna conductor for the AM band and an antenna conductor for the FM band are provided on a vehicle rear window glass, separate adjustment of the reception characteristics of the respective antenna conductors is difficult to achieve. This problem may be overcome by arranging the FM antenna conductor in a vehicle side window glass rather than in the rear window glass. In this instance, there still remains a problem that noise generated by the electric equipment is mixed in the AM antenna arranged in the rear window glass.